Organic EL (Electro-Luminescence) display devices start being put into practical use. One feature of an organic EL display device is being flexible. An organic EL display device includes, in each of pixels, at least one organic EL element (Organic Light Emitting Diode: OLED) and at least one TFT (Thin Film Transistor) controlling an electric current to be supplied to each of the at least one OLED. Hereinafter, an organic EL display device will be referred to as an “OLED display device”. Such an OLED display device including a switching element such as a TFT or the like for each of OLEDs is called an “active matrix OLED display device”. A substrate including the TFTs and the OLEDs will be referred to as an “element substrate”.
An OLED (especially, an organic light emitting layer and a cathode electrode material) is easily influenced by moisture to be deteriorated and to cause display unevenness. One technology developed in order to provide an encapsulation structure that protects the OLED against moisture while not spoiling the flexibility of the OLED display device is a thin film encapsulation (TFE) technology. According to the thin film encapsulation technology, an inorganic barrier layer and an organic barrier layer are stacked alternately to provide a sufficient level of barrier property against water vapor with these thin films. From the point of view of the moisture-resistance reliability of the OLED display device, such a thin film encapsulation structure is typically required to have a WVTR (Water Vapor Transmission Rate) lower than, or equal to, 1×10−4 g/m2/day.
A thin film encapsulation structure used in OLED display devices commercially available currently includes an organic barrier layer (polymer barrier layer) having a thickness of about 5 μm to about 20 μm. Such a relatively thick organic barrier layer also has a role of flattening a surface of the element substrate. However, such a thick organic barrier layer involves a problem that the bendability of the OLED display device is limited.
Patent Document No. 1 discloses a thin film encapsulation structure including a first inorganic material layer, a first resin member and a second inorganic material layer provided on the element substrate in this order, with the first inorganic material layer being closest to the element substrate. In this thin film encapsulation structure, the first resin member is present locally, more specifically, around a protruding portion of the first inorganic material layer (first inorganic material layer covering a protruding portion). According to Patent Document No. 2, the first resin member is present locally, more specifically, around the protruding portion, which may not be sufficiently covered with the first inorganic material layer. With such a structure, entrance of moisture or oxygen via the non-covered portion is suppressed. In addition, the first resin member acts as an underlying layer for the second inorganic material layer. Therefore, the second inorganic material layer is properly formed and properly covers a side surface of the first inorganic material layer with an expected thickness. The first resin member is formed as follows. An organic material heated and vaporized to be mist-like is supplied onto an element substrate maintained at a temperature lower than, or equal to, room temperature. The organic material is condensed and put into liquid drops on the substrate. The organic material in liquid drops moves on the substrate by a capillary action or a surface tension to be present locally, more specifically, at a border between a side surface of the protruding portion of the first inorganic material layer and a surface of the substrate. Then, the organic material is cured to form the first resin member at the border. Patent Document No. 2 also discloses an OLED display device including a similar thin film encapsulation structure.
The thin film encapsulation structure, described in each of Patent Documents Nos. 1 and 2, including an organic barrier layer formed of a resin member that is present locally does not include a thick organic barrier layer, and therefore, is considered to improve the bendability of the OLED display device.
Patent Document No. 3 discloses an OLED display device as described below. A bank layer is formed at the border between pixels. A side surface of the bank layer includes a steeply inclining cliff portion, having an inclination angle larger than, or equal to, 90 degrees, along most of a circumference of each of the pixels, and also includes a slowly inclining portion, having an inclination angle smaller than 90 degrees, along a part of the circumference of each of the pixels. With such a structure, a flattening layer (corresponding to an organic barrier layer in this specification) included in a thin film encapsulation structure formed on the bank layer is suppressed from being delaminated. On the slowly inclining portion shown in FIG. 5 of Patent Document No. 3, the flattening layer has an inclination angle of about 12 degrees. In a conventional thin film encapsulation structure shown in FIG. 12 of Patent Document No. 3, a flattening layer has an inclination angle of about 10 degrees.